Since you asked, I'll give my opinion...
Stick with Ruth, Cobb, Wagner, Johnson, and Mathewson in the prewar realm--preferably Ruth, Cobb, or Wagner. Get a graded (PSA, BGS/BVG, or SGC) period specimen in Ex or better.
In the post war era I recommend Jackie Robinson, Hank Aaron, Willie Mays, and Mickey Mantle. Nrmt or better specimens have a good upside.
For current players I like Jeter, and in the other sports Peyton Manning, Tiger Woods, and Kobe Bryant. Try to get early auto cards of these guys, preferably rookies. On the modern stuff, I like them BVG graded with an auto grade--strive for a 9.5/10 grade.
For example,
Pre War
PSA 5 1915 Cracker Jack Cobb
SGC 70 E95 Wagner
BVG 5 1933 Goudey Ruth, any pose
PSA 7 M116 Johnson
SGC 60 E95 Cobb
SGC 60 E94 Matty
PSA 6 T206 Cobb--any pose, tougher backs a plus
Post War
PSA 8 1954 Topps Aaron
PSA 7 1952 Topps Mays
PSA 7 1953 Topps Mantle
PSA 8 1955 Bowman Mantle
PSA 8 1949 Bowman Robinson
Modern
BGS 9.5/10 2001 Sp Authentic Tiger Woods
BGS 9.5 1996 Topps Chrome Kobe Bryant
BGS 8/10 1998 Playoff Contenders Rookie Ticket Auto Peyton Manning
BGS 9/10 1993 Sp Authentic Rookie Buyback Auto Derek Jeter
Some other cards with good upside imho,
T206 Hindu's and other tougher backs
1914 and 1915 Cracker Jacks
1933 Sportskings Thorpe and Grange
1949 Bowman and 1953 Topps Satchell Paige
1950's Topps and Bowman Mays's and Aarons
High grade 1950's and 1960's football legends (e.g. Jim Brown & Johnny Unitas)
Modern day rookie autograph/patch cards (esp. from Sp Authentic, Bowman and Topps Chrome, Exquisite)
Good Luck, but most of all have fun...
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